1. Field of the Invention
This invention relates to color video cameras and, more particularly, to such cameras which have the function of automatically adjusting the white balance in TTL mode and are capable of obtaining a positive picture from the negative film.
2. Description of the Related Art
The conventional automatic white balance adjusting methods for color video cameras may be divided into two large groups employing (i) the external colorimetry and (ii) the TTL colorimetry. Of these, the automatic white balance adjustment of the TTL type the output signal of the image sensor, based on which the white balance is controlled and, therefore, produces no errors, from which the external colorimetry type suffers, due to the difference in the spectral characteristic between the colorimeter and the image sensor and, in the case of the interchangeable-lens type camera, the change of the spectral characteristic as the lens is interchanged with another one. Accordingly, the latter has widely been utilized.
FIG. 1 is a block diagram illustrating the construction of the conventional automatic white balance adjusting circuit of the TTL type in the color video camera. An image sensor using a CCD or the like produces three output signals of the three primary colors R (red), G (green) and B (blue). Of these, the R and B signals are amplified by white balance control amplifiers 2 and 3, respectively. The color signal G and the outputs of the amplifiers 2 and 3 all are supplied to each of a Y processing circuit 4 for producing a luminance signal (Y signal) and a C processing circuit 5 for producing color-difference signals. The color-difference signals output from the C processing circuit 5 are supplied to a balanced modulator 6 which receives other inputs of color subcarrier signals f.sub.SC1 and f.sub.SC2 of 90.degree. phase difference. The output of the .modulator 6, or the chrominance signal (C signal), and the output of the Y processing circuit 4, or the luminance signal (Y signal), are added in an adding circuit 7. A detecting circuit 8 detects a high-luminance portion (peak value) of the luminance signal output from the Y processing circuit 4. The color-difference signals output from the C processing circuit 5 are also supplied to sample-and-hold (S/H) circuits 9 and 10 and integration circuits 11 and 12. A white balance control circuit 13 controls the gains of the amplifiers 2 and 3 individually through a control voltage conversion circuit 14.
With the camera of such construction, light coming from an object to be photographed forms an image on the image sensor 1 where it is photoelectrically converted into electrical signals. From this image sensor 1, the electrical signals are output for each of the colors R, G and B. Of these, the R signal and the B signal, after their amplitudes have been controlled by the amplifiers 2 and 3 for white balance control respectively, are supplied to each of the Y processing circuit 4 and the C processing circuit 5. The G signal is supplied without further alteration to the Y processing circuit 4 and the C processing circuit 5. And, the luminance signal (Y signal) is output from the Y processing circuit 4, while the color-difference signals R-Y and B-Y are output from the C processing circuit 5.
The aforesaid color-difference signals R-Y and B-Y are supplied to the balanced modulator 6, where they are subjected to balanced modulation by using the color subcarrier signals f.sub.SC1 and f.sub.SC2 of 90.degree. phase difference, and are output as a chrominance signals (C signal). Then, this chrominance signal and the above-described luminance signal (Y signal) are mixed in the adding circuit 7, and therefrom a composite video signal is output, which is supplied to a reproduction circuit (not shown).
Here, the above-described composite video signal is of such a form that the white balance has been adjusted by the amplifiers 2 and 3 for white balance control. That is, in order to adjust the white balance, the high-luminance portion of the luminance signal output from the Y processing circuit 4 is first detected. By using that detection signal, the sample-and-hold circuits 9 and 10 carry out sampling of the color-difference signals R-Y and B-Y. At the same time, the integration circuits 11 and 12 carry out the integration of the color-difference signals R-Y and B-Y. Then, the white balance control circuit 13 controls the gains of the amplifiers 2 and 3 in such a manner that the values R-Y.sub.peak and B-Y.sub.peak of the color-difference signals obtained by the aforesaid sampling in the peak (high-luminance portion) of the luminance signal, or the average values R-Y.sub.Ave and B-Y.sub.Ave of the color-difference signals obtained by integrating them for one picture. Thus, the adjustment is made so as to obtain always optimum white balance. In this connection, it should be pointed out that each of the amplifiers 2 and 3 is controlled through the control voltage conversion circuit 14, and that these amplifiers 2 and 3 are in the form of voltage-controlled amplifiers. Hence, the gain is controlled by the DC voltage applied to the control terminal.
FIG. 2 shows the details of the conventional control voltage conversion circuit 14. The output terminals (ROUT, BOUT) 13a and 13b of a D/A converter in the white balance control circuit 13 are connected respectively through resistors R.sub.1 and R.sub.2 to bleeders of resistors R3 and R4 and resistors R5 and R6 connected across a D.C. electric power source of 5 volts.
From one of the output terminals of the D/A converter in the white balance control circuit 13, say, 13a, a D.C. voltage which takes a value in the range of from 0 to 5 volts depending on the above-mentioned values R-Y.sub.peak and B-Y.sub.peak of the color-difference signals is output. For this D.C. voltage, the width of variation is changed according to the impedance ratio of the resistor R.sub.1 and the bleeder of resistors R.sub.3 and R.sub.4, and the value of the center of the width of variation is determined by the ratio of the resistance R.sub.3 and R.sub.4 of the bleeder, before it is applied to the control terminal (RCONT) 2a of the amplifier 2 for white balance control. Likewise, at the other output terminal 13b of the D/A converter in the white balance control circuit 13, there also appears a D.C. voltage, which, after the width of variation and its central value have been determined by the values of the resistor R2 and the bleeder resistors R.sub.5 and R.sub.6, is applied to the control terminal (BCONT) 3a of the amplifier 3 for white balance control. And, the values of the resistors R.sub.1 to R.sub.6 are previously set so that the central values and the variation widths of these D.C. voltages become optimum in a prescribed color temperature range (usually 3000.degree. K. to 7000.degree. K.).
Because, in such a conventional color video camera as described above, however, the gain of the amplifier for white balance control is fixed, if, as the color negative film is shot to obtain a positive picture, the picture is inverted, the gain of the amplifier for white balance control does not become a proper value. This leads to a problem in that the optimum white balance adjustment cannot be assured.
Also, in the conventional color video camera described above, the luminance signal output from the Y processing circuit 4 is supplied as it stands to the above-described detecting circuit 8 so that the high-luminance portion is detected. When the color negative film is shot to obtain a positive picture, therefore, the high-luminance portion of the luminance signal cannot reliably be detected. This contributes to a problem of failing to attain an optimum white balance adjustment.
Furthermore, in the conventional color video camera described above, the color signals output from the image sensor 1 are supplied directly to the amplifiers 2, 3 and the processing circuits 4, 5. When a color negative film is shot to obtain the positive picture, the influence of the film base which is a colored orange system cannot be removed. This contributes to a problem in that the white balance cannot accurately be adjusted.